Quadruply ridged waveguide and horn antenna



July 29, 1969 R. E. FRANKS QUADRUPLY RIDGED WAVEGUIDE AND HORN ANTENNA Filed Aug. 8, 1966 2 Sheets-Sheet 2 Fig- 7 FREQUENCY (MHZ) INVENTOR RAYMOND ELFRANKS BY MgLQLML ATTORNEY United States Patent ABSTRACT OF THE DISCLOSURE A hollow waveguide having two pairs of diagonally opposed ridge members which extend from the waveguide wall to the waveguide center and in which each ridge member includes an enlarged end portion disposed ad jacent the waveguide center and a narrow support fin for supportively connecting the enlarged end portion to the waveguide wall.

This invention relates in general to the transmission of electromagnetic wave energy, and relates more particularly to structures for transmitting electromagnetic wave energy over a Wide bandwidth.

It is well-known in the art to utilize rigid Waveguides to increase the bandwidth capacity of the waveguide member. Such ridged waveguides are usually formed by adding a central ridge which extends into the waveguide member. This ridge portion has the effect of increasing the capacitance between top and bottom walls of the waveguide, and hence lowers the cut-off frequency of the guide.

While such prior art devices have been useful in increasing the bandwidth available, there are a number of problems associated therewith where a very small gap is required. One problem is that it is very difiicult to accurately fabricate such ridged members where the required gap between the opposed surfaces of the ridge members is quite small, as it should be when a maximum lowering of the cut-off frequency is desired. For example, bandwidths of the order of to 1 require a gap about onetwentieth of the height of the Waveguide, and this latter ratio decreases as the square of the bandwidth increases.

An additional problem is that as the gap width decreases, the effective impedance of the guide also decreases, thus making it difiicult to obtain a satisfactory impedance match with higher impedance elements which maybe connected to the waveguide, such as coaxial cable.

In accordance with the present invention, there is provided a novel ridged waveguide structure which is effective to increase the bandwidth capacity of the guide without requiring difiicult fabrication procedures in manufacturing the guide. Further, the structure is such that its impedance is higher than that of the prior art devices, thus facilitating a proper impedance match with elements which are to be connected to the guide.

In this invention, the guide is formed of a plurality of ridge members which extend toward the center of the guide, and each ridge member includes a large end portion disposed near the center of the guide and a smaller, fin-type support connecting the large portion to the outer wall of the guide. The smaller, fin-type support is in contrast to the support members of the prior art, which are of the same general cross-sectional size as the end p0rtions at the center of the guide.

In the present invention, this fin-like support has the effect of increasing the impedance between adjacent ones of the support members by increasing the current path therebetween. This increased impedance has the effect of permitting an increase in the bandwidth available with a Patented July 29. 1969 given gap or separation of the large end members, since the bandwidth capacity of a given ridged guide is a function of the ratio of the impedance, Z of the larger portion of the guide to the impedance, Z02, across the gap between the ridge members. Thus, by increasing Z as a result of the greater current path between fin-type supports, the present invention effectively increases the above ratio to thereby increase the bandwidth capability of the guide for a given gap separation. Alternatively, for a given bandwidth range, the structure of this invention may employ a larger gap than that possible in the prior art devices, thus facilitating the manufacture of the structure.

An additional effect of this increased impedance is to increase the overall impedance of the guide member, thus facilitating matching this impedance to that of an element to be connected to the guide, such as a coaxial cable or the like. A further effect of the present invention is to reduce the size and weight of the ridged guide structure.

In the preferred embodiment of this invention, the ridged guide is adapted to transmit two orthogonal linearly polarized waves. This dual orthogonal polarization output may be employed, for example, for polarization diversity to minimize polarization losses. In these circumstances, the novel waveguide is provided with four ridge members therein, one pair for each of the linearly polarized waves, and with each pair of ridges displaced from each other to maintain the desired orthogonal relationship. Each of the ridge members is formed as described above, with a larger end portion near the center of the guide and a smaller fin-type support member connecting the end portion to the guide wall.

Where the Waveguide of this invention is to be used to couple the transmitted wave energy to a horn radiator or the like, the horn employed may have a structure similar to that of the ridged guide member. Thus, the quadruply ridged guide cross-sectional dimensions may be expanded gradually to maintain the required low frequency cut-off and to provide an impedance transformation to free space, resulting in a horn configuration which resembles that of the guide structure.

It is therefore an object of this invention to provide a novel ridged waveguide structure having improved bandwidth capacity.

It is a further object of the present invention to provide a ridged waveguide structure having an increased impedance Z for a given gap between the ridge members to thereby increase the impedance of the guide and increase its bandwidth capacity.

It is a further object of the present invention to provide a ridged waveguide having a plurality of ridge members therein, each ridge member having a large end portion disposed near the center of the guide and having a smaller, fin-type support connecting the large end portion to the guide wall.

It is an additional object of this invention to provide a ridged waveguide having four ridge members therein for the transmission of two orthogonal linearly polarized waves.

It is an additional object of this invention to provide a ridged waveguide member which is easier to manufacture and lighter in weight than those of the prior art.

Objects and advantages other than those set forth above will be apparent from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is a schematic diagram of a conventional doubly ridged waveguide illustrating the impedances Z and Z FIGURE 2 is a perspective view, partly in cross section, illustrating one embodiment of the quadruply ridged waveguide of this invention;

FIGURE 3 is a perspective view, partly in cross section, illustrating an alternate embodiment of the waveguide of this invention;

FIGURE 4 is a cross-sectional view of the waveguide shown in FIGURE 3, illustrating the connection of two coaxial lines thereto;

FIGURE 5 is a view along line 5--5 of FIGURE 4 showing further details of the connection of the coaxial lines and a termination chamber for terminating the line;

FIGURE 6 is a perspective view of an antenna horn formed by expanding the ridged waveguide of FIGURE 3; and

FIGURE 7 is a VSWR diagram obtained with an antenna horn formed as shown in FIGURE 6.

Referring to FIGURE 1, there is shown a cross section of a typical doubly ridged waveguide to illustrate the impedances Z and Z The waveguide comprises side wall sections 11a and a central section where the walls 11b form ridges to provide closely spaced opposed surfaces. As indicated above, such a construction is wellknown in the art to increase the bandwidth capacity of a waveguide by virtue of the capacitive effect between the ridge members 11b. As indicated in the drawing, the impedance across the gap formed by these ridge members may be designated Z while the impedance in the larger wall section is represented by Z The bandwidth capacity of the waveguide is a function of the ratio of Z to Z and by increasing Z or decreasing Z this bandwidth can be increased.

One embodiment of the present invention is illustrated in FIGURE 2, where a waveguide member 21 of circular cross section is provided with a plurality of ridge members. These ridge members include large end portions 22a, 23a, 24a and 25a which are disposed in the central portion of waveguide 21. Each of end portions 22a, 23a, and 24a and 25a may be formed as pie-shaped segments which taper in cross section to essentially a point toward the center of guide 21, and the adjacent walls or edge portions of each of these members are parallel to each other to form wave transmission paths.

Each of the larger end portions is connected to the wall of guide 21 by means of fin-like support members 22b, 23b, 24b and 25b which may be of a suitable material such as brass. As shown clearly in FIGURE 2, these fin like support members are considerably thinner in cross section than the larger end portions 22a, 23a, 24a and 25a. This reduced cross section is effective to produce the increased impedance Z for the structure, since it can be seen that the current path between adjacent support members 22b, 23b, 24b and 25b is considerably longer than if the support members were of the same cross-sectional shape as the end portions 22a, 23a, 24a

and 25a.

' This increased current path in turn results in a higher value of impedance Z in this portion of the waveguide. Thus, for a given gap separation of the end portions 22a,

23a, 24a, 25a, this increased impedance Z results in a higher ratio of Z to Z thereby resulting in a higher bandwidth capacity of the guide, as discussed above. Alternatively, for a given bandwidth capacity of the guide, the separation or gap between end portions 22a, 23a 24a, 25a may be made larger than that required in the prior art devices, thus facilitating the fabrication of the waveguide structure.

As a further advantage of this structure, this increased impedance Z produces an increased impedance for the waveguide as a whole, thereby facilitating matching the guide impedance to that of elements to be connected thereto, such as coaxial cable. For example, with the structure of this invention, an impedance of 50 ohms may be obtained for a waveguide with ten to one bandwidth which presents a good match to the 50 ohm impedance of most coaxial cables, as contrasted to the 20 ohm impedance obtainable with prior art ridged waveguides.

It will also be seen that the structure of this invention is considerably lighter in weight and smaller in size, by virtue of the use of the fin-like support members, than it would be if these support members were of the same crosssectional configuration as the end portions 22a, 23a, 24a, 25a.

FIGURE 3 illustrates an alternate embodiment of the waveguide in accordance with this invention, in which the ridged members have a different configuration from that shown in FIGURE 2. In FIGURE 3, waveguide tube 21 has a circular cross section, as before, and is provided with four ridge-forming members therein. These ridge members have end portions 32a, 33a, 34a and 35a'which are generally V-shaped, with the point or apex of the V directed toward the center of guide 21. The adjacent walls or edge portions of end portions 32a, 33a, 34a and 35a are generally parallel to each other, as in the embodiment of FIGURE 2, to form wave transmission paths.

The end portions 32a, 33a, 34a, 35a are supported within guide 21 by fin-like support members 32b, 33b, 34b and 35b which are secured to their respective end portions at the back of the apex of the V. As in the embodiment of FIGURE 2, the fin-like support members 32b, 33b, 34b, 35b serve to reduce the impedance Z in the waveguide, thus permitting an increase in the ratio Z /Z to thereby produce an increased bandwith capacity for the waveguide for a given gap separation between end portions 32a, 33a, 34a, 35a. This increased impedance Z also increases the overall impedance of the waveguide to facilitate matching the impedance of a coaxial cable or the like which is connected thereto.

As indicated above, waveguides such as shown in FIG- URES 2 and 3 are particularly adapted for the transmission of two orthogonal linearly polarized waves. One arrangement for coupling such waves to the waveguide is illustrated in FIGURES 4 and 5. In the figures, the ridged waveguide structure of FIGURE 3 is used to transmit two orthogonal linearly polarized waves supplied from coaxial lines 41 and 42. These coaxial lines have inner conductors 41a, 42a, which are maintained separated from the respective outer conductors 41b, 42b, as is wellknown in the art.

Coaxial lines 41, 42 must be brought into the same etfective cross-sectional plane of the waveguide to achieve equal performance for the two linear polarizations. Accordingly, the coaxial lines are displaced degrees from each other around the periphery of waveguide 21 and extend therethrough into contact with the ridge members. Inner conductor 41a of line 41 is connected to the apex of end portion 33a, while the outer conductor 41b is connected to the opposite end member 35a. Similarly, inner conductor 42:: is connected to the apex of end portion 34a, while outer conductor 42b is connected to the opposite end portion 32a. The points of coupling of the coaxial lines are as close together as possible, and as indicated clearly in FIGURE 5, these points of connection are offset from each other along the length of waveguide 21 only by an amount suflicient to permit the required connections to be made.

To provide proper coupling for the coaxial lines, a termination chamber 44 is provided as shown in FIGURE 5. Termination chamber 44 has a length L which is less than one-half the wavelength A of the highest frequency to be transmitted in the waveguide. The termination chamber acts as a characteristic high value of impedance, rather than as an absorber, to provide a negligible shunting effect on coupling between coaxial lines 41, 42 and the ridged waveguide structure. Cylindrical conductors of the same dimensions as conductors 41b and 42b may be added to the fins 33b and 34b to provide a symmetrical means of terminating the four fin-like support members.

Where the quadruply ridged waveguide of this invention is to be coupled to an antenna horn, a suitable horn may be formed by tapering the ridged waveguide dimensions up to the horn aperture dimensions. Such a horn is illustrated in FIGURE 6, and is produced by tapering the waveguide structure of FIGURE 3 from the throat of the horn up to the horn aperture dimensions. The finlike support members 32b, 33b, 34b, 35b and the V-shaped end portions 32a, 33a, 34a, 35a of the ridged waveguide are tapered outwardly into corresponding portions 32b, 33b, 34b, 35b and 32a, 33a, 34a, 35a in the horn 48. This expansion of the ridged cross-sectional dimensions is sufliciently gradual to maintain the required low frequency cut-off and to provide an impedance transformation from the waveguide to free space. The rate of flare of the horn must be made sufliciently large so that the horn length is maintained within practical limits. This may result in a phase taper across the horn aperture which must be corrected by a lens, such as a real dielectric lens.

A VSWR diagram over a twenty to one frequency band for a horn constructed as shown in FIGURE 6 is illustrated in FIGURE 7. From this diagram it will be seen that the VSWR will be less than 2.5 to 1 over 90 percent of the band, and that the VSWR will be below 6.0 to 1 over the remainder of the twenty to one frequency band.

Although the above description has been in connection with waveguides of circular cross section, it will be apparent to those skilled in the art that the invention may also be applied to waveguides of rectangular cross section by determining the proper dimensions of equivalent rectangular guides to achieve the desired increase in the ratio of waveguide height to gap between the ridges.

While the above detailed description has shown, described and pointed out the fundamental novel features of the invention as applied to various embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated may be made by those skilled in the art, without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is:

1. A ridged waveguide comprising:

a hollow outer conductor; and

two pairs of opposed ridged members disposed within said hollow outer conductor, said ridged members being conductively coupled to the wall of said outer conductor and extending inwardly towards the hollow central portion of said outer conductor, the crosssectional area of said ridged members being less immediately adjacent the wall of said outer conductor than in the hollow central portion of said outer conductor.

2. A ridged waveguide in accordance with claim 1 in which each of said ridged members comprises a fin-like support member extending from the wall of said outer conductor and terminating in a larger end portion disposed in the hollow central portion of said outer conductor.

3. A ridged waveguide comprising:

a hollow outer conductor; and

two pairs of ridge members disposed within said hollow outer conductor, each of said ridge members being conductively coupled to the wall of said outer conductor and extending inwardly toward the hollow central portion of said outer conductor, the ridge members of each said pair being disposed diametrically opposite each other within said outer conductor, each of said pairs of ridge members being displaced 90 degrees from the other of said pairs, the cross-sectional area of each of said ridge members being less immediately adjacent the wall of said outer conductor than in the central portion of said outer conductor.

4. A ridged waveguide in accordance with claim 3 in which each of said ridge members has edge portions, the edge portions of adjacent ones of said ridge members being parallel to one another to define a plurality of gaps between said ridge members.

5. A ridged waveguide in accordance with claim 3 in which each of said ridge members comprise a fin-like support member extending from the wall of said outer conductor and a larger terminal end portion disposed in the hollow central portion of said outer conductor.

6. A ridged waveguide in accordance with claim 5 in which each of said larger terminal end portions has a generally V-shaped cross section with the apex of the V directed toward the center of said outer conductor, and in which said fin-like support members are connected to said larger terminal end portions at the back of the apex.

'7. A ridged waveguide in accordance with claim 6 in which each of said ridge members has edge portions formed by the legs of the V-shaped terminal end portions and in which adjacent ones of said edge portions are disposed parallel to one another to define the plurality of gaps between said ridge members.

8. Apparatus for transmitting two orthogonal linearly polarized electromagnetic waves, comprising:

a hollow outer conductor;

two pairs of ridge members disposed within said conductor, each of said ridge members being conductively secured to the inner surface of said outer conductor and extending inwardly toward the central portion of said outer conductor, the ridge members of each said pair being disposed opposite each other within said outer conductor, each of said pair of ridge members being displaced degrees from the other of said pairs, the cross-sectional area of each of said ridge members being less immediately adjacent the inner surface of said outer conductor than in said central portion of said outer conductor;

first coaxial line means extending through said outer conductor and connected to one of said pairs of ridge members; and

second coaxial line means extending through said outer conductor and connected to the other one of said pairs of ridge members.

9. Apparatus in accordance with claim 8 in which said first and said second coaxial line means enter said conductor in substantially the same cross-sectional plane.

10. Apparatus in accordance with claim 8 including horn antenna means connected to said outer conductor and having a throat at one end and an aperture at the other end, said antenna means having a plurality of ridge members disposed within said throat corresponding to said ridge members in said outer conductor, the ridge members of said outer conductor being conductively coupled to the ridged members of said antenna means, and the last mentioned ridge members gradually increasing in separation and decreasing in height along the length of said antenna means to merge into the inner surface of said antenna means and thereby produce an impedance match to free space at said aperture.

References Cited UNITED STATES PATENTS 2,777,906 1/1957 Shockley 333- 3,090,020 5/ 1963 Lunden 333--95 FOREIGN PATENTS Ad. 70,819 2/ 1959 France.

449,247 10/1944 Canada.

ELI LIEBERMAN, Primary Examiner U.S. Cl X.R. 333-95 I 

